Enhanced separation of silicon and aluminum in fly ash via sodium-alkali regulation

Silicon-aluminum separation is crucial for addressing the challenge of achieving high-value utilization of fly ash. An effective approach for separating silicon and aluminum from fly ash was developed via controlled sodium regulation during soda roasting, followed by selective acid leaching. Phase diagram analysis, together with viscosity and activity calculations based on the major fly ash constituents, guided the optimization of roasting conditions to obtain a nepheline-dominated phase system. Experimental results confirmed that precise sodium dosage, determined through thermodynamic calculations, plays a key role in achieving high aluminum extraction efficiency exceeding 80%. Phase and microstructural characterization (XRD, SEM-EDS, TEM-EDS, FTIR, XPS, NMR) demonstrated that roasting at 800 °C with regulated sodium addition facilitated the formation of nepheline and NaAlO₂. These phases, associated with a structural transition toward Q⁴(0Al) and Q⁴(1Al) dominated frameworks, enhanced selective aluminum leaching during subsequent acid treatment. The close consistency between theoretical predictions and experimental findings provides a reliable basis for fly ash valorization through efficient silicon and aluminum species separation.